Electrocoagulation removal of Pb, Cd, and Cu ions from wastewater using a new configuration of electrodes

A new configuration of aluminum electrodes has been performed in an electrocoagulation reactor (ECR) to remove toxic metals from synthetic wastewater. The ECR contains four concentric-cubic electrodes that were connected to the DC power supply with a bipolar mode. The ability of this reactor to eliminate 200 ppm Pb, 200 ppm Cd and 200 ppm Cu from wastewater was investigated under the effect of pH (4–10), applied current (0.2–2.6 A), and the reaction time of (4–60 min). Two grams of NaCl were added to each experiment to enhance the electrical conductivity and minimize the passivation of cathode surfaces. The experiments, analysis, and optimization were conducted using response surface methodology type Box-Behnken design (RSM-BBD) and the Minitab-statistical software program. The highest elimination of heavy metals was: Pb-99.73%, Cd-98.54%, and Cu-98.92% at pH 10, 1.4 A of the applied current, and 60 min of the reaction time. The total real consumption of anodes under these conditions was 0.55 g, and the energy consumption was 12.71 kWh/m3. All reactions of metal removal that occurred in the present EC reactor obey the kinetic of a first-order reaction. Thermodynamics parameters of present electrocoagulation removal of heavy metals indicate an endothermic, spontaneous nature, and random irregularity at the liquid-solid interaction. The highest values of removal efficiencies and the considerably lowest values of energy and electrode consumption proved that the electrocoagulation technology applies in wastewater treatment containing toxic metals.• The anode electrodes were perforated to decrease the amount of electrode consumption, while the cathode electrodes were not perforated.• The new EC reactor eliminated Pb-99.73%, Cd-98.54%, and Cu-98.92% of 200 mg/l of each metal at pH 10, applied current of 1.4 A, and reaction time of 60 min. Moreover, the consumption of energy and electrodes was significantly low.• The performance indicator (R2) of the studied responses was higher than 0.95.


Overview
In recent decades, rapid industrialization and modern agricultural practices, as well as unplanned urbanization, have affected the environment with numerous contaminants that threaten humanity [1][2][3][4] . Water contamination by heavy metals produced from several activities, such as metal plating, ore mining, fertilizer, batteries, paper, paints, pesticides, etc. ( Table 1 ), has widely received the attention of many scientists and researchers. A batch EC containing bipolar aluminum electrodes was used by Assadi, et al. (2015) to remove lead from wastewater under the effect of the electrolysis time (5-30 min) and current density (11,22,33 A/m2), lead concentrations (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) ppm, and pH (5)(6)(7)(8)(9). The highest removal of lead attained was 94% at pH 7, 33 A\m2 of current density, and 30 min of the reaction time [5] . Abdul Rehman, et al. (2015) conducted a continuous EC reactor containing bipolar aluminum and iron plane electrodes to eliminate 105 ppm of Cu, 110 ppm of Ni, and 63 ppm of Pb from wastewater under the impact of current density (0.0070-0.040) A/cm2, retention time (20-120 s), pH (3)(4)(5)(6)(7)(8)(9) and (4-24 mm) of the distance between electrodes. This work attained 95% of metals removal efficiency, 0.026 A/cm2 of the current density, and a solution pH of 6.32 [6] . Al-Nuaimi and Pak (2016) attained 91.72% of chromium removal efficiency at 14 mA/cm2 of current density, pH = 6.7, 1 cm of the distance between electrodes 15 g/l of KCl, and 90 min of reaction time [7] . Abdul Majeed, et al. (2018) employed a batch EC reactor containing two aluminum electrodes as the anode and copper-cathode to eliminate nickel from wastewater under the influences of voltages and the reaction time. They achieved the highest removal of Ni at 5 Vs within 76 min [8] . While Patel and Parikh (2020) removed chromium (VI) from wastewater using copper electrodes in an EC reactor. The highest removal was 98.15% at 41.32 A/m2 of current density, pH 7, and 1.4 cm of the distance between the electrodes [9] . It will show later other previous studies whose concerned with removing heavy metals from wastewater in a summarized table.
Heavy metals are non-degradable and not converted to more simple forms, like organic pollutants. They are also toxic due to their effects on living organisms, easy bioaccumulation in the food chain, and comprehensive sources [1 , 2 , 10] . The wastewater is classified as water-toxic metal pollution when it is polluted by high-density elements with (63.5 to 200.6) atomic weight or an atomic number greater than 20 and a weight density greater than 5 g/cm 3 [1-3 , 11] . Heavy metals such as, lead (Pb), cadmium (Cd), zinc (Zn), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), manganese (Mn), silver (Ag), platinum (Pt), and arsenic (As) are released daily from different industrial and domestic activities, causing a serious hazard to the human health, natural plant, and fauna [2 , 12-14] . Table 2 shows the major sources, health effects of some toxic metals that have been treated in the present work, and their permission limits on drinking water as documented by the World Health Organization (WHO) [1 , 13 , 15] . Table 1 The main sources of heavy metal contamination in the environment [13] .  Table 2 The main effects, sources, and permission limits of some toxic metals [1 , 13-15] .  As revealed in Table 2 , toxic metals must be eliminated from water using effective treatment methods before being discharged into the environment because they are non-biodegradable compounds.

Electrocoagulation
Electrocoagulation (EC) is a type of electrochemical technique including essential advantages, such as versatility, costeffectiveness, selectivity, safety, low value of sludge production, considerable removal efficiency, and energy efficiency [32][33][34] . The mechanism of EC depends on the electrochemical production of metallic ions from electrodes (such as aluminum that is used in this investigation) to form coagulants (Aluminum hydroxide Al(OH)3) which are required to remove pollutants from samples Fig. 1 ). Eqs. (1) -(3) show the chemical reactions occur in the EC reactor at the anode and cathode [32 , 35 , 36] : At the anode: At the cathode: Formation of coagulants: Removal efficiencies (Y i ) of heavy metals, i.e. (Y Pb %), (Y Cd %), and (Y Cu %), were estimated using Eq. (4) as follows [37] : where C 0 and C t are the concentrations of metal at time (0) and (t), respectively. In addition, the consumption of energy (kWh/m 3 ) and the theoretical consumption of electrodes were measured by using the Eqs. (5) and (6) [31 , 32 , 36] : where V is the cell voltage in volt, I will be the current intensity in ampere, t is the reaction time in hour, and V R is the volume of wastewater sample in cubic meter.   where M is the M.wt of aluminum electrodes, Z equals 3 (for aluminum), and F is Faraday's constant, which equals 96,485.34 Columb/mol. The actual consumption of aluminum electrodes is determined depending on the weight difference of aluminum electrodes before and after each run.
Hence, the work aims to assess the multi-heavy metal removal efficiency and determine the consumption of energy and electrodes under the impact of specific operating conditions by employing a new configuration of multi-concentric-cubic aluminum electrodes. The reactor shape and the configuration of the electrodes are the keys to estimating the performance of each electrocoagulation reactor [31] . The response surface methodology type Box-Behnken design (RSM-BBD) and Minitab soft program were used for experimental design and analysis.

Apparatus
Four concentric aluminum cubic electrodes with different dimensions ( Fig. 2 and Table 3 ) are employed in a batch EC reactor. The anode electrodes (AE-1 and AE-3) have been perforated with a total active area of 360 cm 2 , while the cathode electrodes (CE-2 and CE-4) were non-perforated with a total active area of 512 cm 2 .
However, the XRD-test result of aluminum electrodes is shown in Table 4 as follows: This study has conducted the EC experiments at room temperature using a glass cell with a total volume of 3000 ml and 2000 ml of the experimental volume. It provided a constant stirring speed of 300 rpm for the EC reactor containing solution using a magnetic stirrer (Model: ALFA company: HS-860; 0-3000 rpm). The electrodes were connected to a DC-power supply (Model: DC power supply type SYADGONG, China) and arranged in a bipolar connection mode. Solution pH was adjusted to the designed value using 0.1 N HCl and 0.1 N NaOH. Hence, the value of pH was measured before and after each experiment using an electronic pH meter (Model: ATC company, China). The electrolyte used for support was an NaCl solution of 2 g/L. The synthetic wastewater was prepared by After each run, these electrodes were polished with soft sandpaper and washed with dilute HCl and distillate water to remove the oxide layer, and then dried to be weighed after electrocoagulation. Final concentrations of toxic metals were determined using atomic absorption spectroscopy (type-AA-7000F, Shimadzu, Japan) and the removal efficiency of these metals was calculated using Eq. (4) .

Experimental design
To approach a few experimental processes involving the interaction of the studied variables and modeling of parameters of the studied responses, response surface methodology (RSM) type Box-Behnken Design (BBD) and Minitab program were used to design the EC experiments and analyze the results.
In  Table 5 and the experimental design matrix from RSM-BBD is explained in Table 6 , which comprises 52 actual conditions and the core results of the studied responses.

Results and discussions
Removal efficiency of Pb, Cd, and Cu metals Table 6 listed the removal efficiencies of toxic metals based on the aluminum-concentric cubic electrodes and considered all factors. In Table 6 , columns 2, 3, 4, 5, 6, and 7 indicate the actual values of the operating variables; initial concentrations of Pb, Cd, and Cu metals (ppm), solution pH, electric current (A), and the reaction time (min). The NaCl electrolyte was used to assist in the removal of toxic metals from synthetic wastewater. The EC technology is dependent on the solution pH of the wastewater during the periods of the experiments. Solution pH influences the formation of metallic electro-coagulants, and the initial solution pH influences the EC performance [32 , 38-40] . The type of electrode material, especially the anode, affects the performance of the electrocoagulation reactor because it determines the kind of cations that are released into the solution that plays a significant role in the formation of flocs [32] .
As shown in Fig. 3 , the removal efficiencies are increased when the initial pH of wastewater is increased until the higher value of pH because of the formation of undesired hydroxo-complexes such as [Al(OH) 4 − ] and [Al(OH) 5 2 − ] which are not useful to form electro-coagulants and then affect the treatment process [41] . However, the Cd removal efficiency kept increasing even in the higher basic solution. These results agreed with [15 , 32 , 42] , which indicated the significant effect of solution pH value on the removal efficiency of heavy metals.
It is evident that the removal efficiency of each metal differs in its behavior based on solution pH, which is associated with the formation of hydroxyl ions at the cathode and Al ions from the anode as a natural result of the continuous passing of electricity through the aluminum electrodes. The formation of electro-coagulants on one side and the variation in the pH value on the other will influence the metal removal efficiency value. The difference in the current value supplied to the EC cell during the electrocoagulation process of each experiment clearly established this.
The mathematical equations of curves presented in Fig. 3   The current intensity has a significant impact on the EC process because it regulates the formation of electro-coagulants depending on the anodic dissolution based on Faraday's law. It is the main parameter of the performance of all electrochemical methods, which is the more effective parameter to control the rate of reaction in the electrochemical cell. Therefore, the removal efficiencies of toxic metals are increased when the current intensity is raised [43][44][45] . The influence of applied electric current on the EC cell is especially essential since the release of hydrogen and oxygen rate influences the mechanism of the EC process [46][47][48] . As observed, when the applied current raised from 0.2 to 2.6 A, toxic metals elimination increased, as shown in Fig. 4 and their mathematical relations Eqs. (10) - (12) . The excessive increase of current will increase the ohmic drop and subsequently affect the EC performance. As seen in Fig. 4 , the Cd metal was more sensitive to the continuous increase of applied current compared to the other two metals that kept eliminated until a higher value of current. These findings are the same as [42 , 49 , 50] . It is obvious that the continuous supplying of electric current through electrodes has to control the amount of metals ions released from them and, consequently, enhancing the The operating variable of the reaction time is extremely affecting the quantity of Al released from the concentric electrodes, which react with OH ions to form the electro-coagulants that determine the toxic metals' removal efficiencies [51][52][53] . Fig. 5 reveals the effect of these variables on the removal efficiencies where all metals have removed when the reaction time increased, but Cu removal was rapidly increased and then early decreased compared to other metals that have minimized after a while. This behavior depends on the ability of the attraction force between each metal and electro-coagulants formed throughout the reactor. These results are like those of [4 , 32 , 35] . The mathematical correlations between the removal efficiencies and the reaction time (X 6 ) are as follows Eqs.
Y Cu % = 4 . 29 X 6 − 0 . 05 X 2 6 ( 100 ppm Pb ; 100 ppm Cd , 100 ppm Cu ; pH 7; 1 . 4 A ) The highest removal efficiencies of toxic metals were Pb-99.73%, Cd-98.54%, and Cu-98.92% at pH 10, applied current of 1.4 A, and reaction time of 60 min. The total actual consumption of electrodes under these conditions was 0.55 g and the energy consumption was 12.71 kWh/m 3 . These results show that the EC has attained the maximum removal efficiencies of toxic metals with low consumption of electrodes and electrical energy.

Analysis with RSM-BBD
The RSM is a statistical method that is useful for designing experiments, analyzing, and optimizing the studied variables and responses [32 , 54 , 55] . The validation of the adequacy of the mathematical models estimated is achieved by using the analysis of variance (ANOVA). This test is used to analyze the regression models and fit them to the data in order to estimate the misleading findings that may influence the accuracy of the developed regression models [56 , 57] . Tables 7-10 show the ANOVA test results for Pb removal%, Cd removal%, Cu removal%, and energy consumption. These results show that the studied responses have a significant impact ( p < 0.05) on the removal of heavy metals, which means that the estimated model is significant at 95% of the probability level. However, Table 11 lists the mathematical models of the studied responses and their regression coefficients, where the highest values of these coefficients mean that the quadratic models are significant [56] .
As shown in Tables 7 -9 and based on F-values, the initial concentrations of Pb, Cd, and Cu were the most important variables in Pb, Cd, and Cu removal efficiencies, respectively. While the current applied was the most essential variable in the energy consumption response, as shown in Table 10 . The large value F-indicator means that the mean square contributed by the regression model is much higher than the mean square error [57] .
As revealed in Table 6 , the amount of electrode consumption was different for each anode because the AE-1 was directly connected to the electric source while the AE-3 was in the state of the bipolar electrode. Fig. 6 (a, b, and c) illustrates the consumed amount of each anode required to attain the removal efficiencies of toxic metals.
As observed in Fig. 6 , the consumption of the AE-3 was similar in behavior for the removal of metals. The consumption of AE-3 was slower at the low value of removal efficiency, but then it was consumed rapidly until the highest remediation of metals was attained. Each metal removal had a different behavior of the AE-1 consumption. The Pb-removal% consumed a larger amount of AE-1 if compared to others, especially at the higher value of removal efficiency. However, the highest elimination of Cd metal depended on consuming AE-3 more than the consumption of the AE-1 electrode. The irregular behavior presented in Fig. 6 maybe refers to the formation issue of an oxide layer at some locations of electrodes. This layer occurred when the NaCl electrolyte could not remove this passivation because of the huge amount of tiny gas bubbles formed on the surface of the electrodes. As shown in Fig. 7 , the lowest removal efficiency of Pb metal had the lowest total consumption of both anodes, but the highest Pb-removal efficiency was inverse. However, the highest removal efficiency of Cd metal has the highest total consumption of anodes. But the situation for Cu metal was in between. Interpreting these behaviors may refer to as the degree of the attractive force that occurred between the pollutants and the electro-coagulants formed [36 , 58-60] .   Fig. 7. Removal efficiencies of toxic metals vs. the total consumption of anodes.

Kinetic study
The order of reaction that occurs throughout any electrochemical reactor should be investigated. The kinetic modeling for the present configuration of electrodes is provided to obtain the rate constants of the EC process. First and second-order equations are presented in Eqs. (20) and (21) as follows: where C i and C t are the concentration of each metal at initial and time t, respectively, k 1 and k 2 are the rate constant for first order (min − 1 ) and second order (m 3 . mol − 1 . min − 1 ), respectively, and t is the reaction time in minutes. The results presented in Table 12 revealed that Pb, Cd, and Cu metals removal obeyed the kinetic of first-order reaction in their behaviors based on the regression coefficient (R 2 ). These results gave an additional advantage for the present EC reactor because the kinetic of second-order reaction is more slower and complicated compared to the first-order reaction.

Thermodynamic parameters
The values of thermodynamic parameters have been estimated based on the temperature variation measured periodically throughout the EC reactor under the optimal conditions. Eq. (22) lists the estimated relation between the equilibrium constant (K d ) and the reciprocal of the solution temperature.

ΔG = ΔH − TΔS
Based on Fig. 8 , the value of H is obtained from the slope of this line which equals 104.229 kJ/mol. The values of G was negative, and S was positive. The present EC process is endothermic, spontaneous nature, and random of irregularity at the liquidsolid interaction.

Optimization with RSM-BBD
RSM-BBD has estimated the optimum conditions for the studied operating variables and the required responses using the Minitabstatistical software program. The optimization of Pb, Cd, and Cu-ions were chosen within the ranges, and it maximized the studied responses of removal efficiencies. However, the optimization of energy consumption and total consumption of electrodes was minimized. Fig. 9 and Table 13 show the optimization process of the studied variables and responses by considering all the operating variables.  Fig. 9. The optimization of the studied variables.

Table 11
The quadratic models for the studied responses.  Fig. 10. The optimization of the studied variables for total amounts of heavy metals.

Table 13
The values of the optimal studied variables and the required responses.   Since the target of any treatment process is to remove the initial concentration of all pollutants found in the wastewater, Fig. 10 and Table 14 provide the observed values of the studied responses when all concentrations of heavy metals are proposed to be the optimal values. Table 15 summarizes some previous works that concerned the removal of heavy metals from wastewater using different configurations of electrodes. This table lists the type of metal used for electrodes regardless of the configuration of them, the optimal values of the operating variables, and the highest removal achieved.

Cost estimation
The estimation of the operating cost is remarkably essential to assess the performance of the present design of the EC reactor used to remove toxic heavy metals from wastewater. The main issue of this subject is the estimation of electrodes and electrical energy consumption that should be taken into consideration. Eq. (25) is used to estimate the cost of the present consumption of electrodes and energy.

Total cost = A 1 × ENC [ kWh ∕ m 3 ] + A 2 × Real consumption of electrodes
where A 1 and A 2 are the prices of unit electrical energy [$/kWh] and unit weight of aluminum [$/g], respectively. At the research time, they equal 0.013 $/kWh and 2.74 × 10 -3 $/g, respectively, according to the local price.
Since the real consumption of electrodes under the optimal conditions was 0.55 g and the energy consumption was 12.71 kWh/m 3 , the total cost was 0.167 $ per each cubic meter of the treated wastewater.

Conclusions
Industrial activities are discharging huge amounts of metal wastewater into the environment without efficient treatment. This study employed an electrocoagulation reactor to eliminate multi-toxic metals from synthetic wastewater using four concentric cubic electrodes made of aluminum. The anodes have a perforated shape, while the cathodes do not. This work has studied several operating variables using the RSM-BBD design method to evaluate the responses of pollutants removal efficiencies and the consumption amounts of energy and electrodes. The highest removal efficiencies of Pb, Cd, and Cu metals were 99.73%, 98.54%, and 98.92%, respectively, with an energy consumption of 12.71 kWh/m 3 and electrodes consumption of 0.55 g at pH 10, applied current of 1.4 A, and reaction time of 60 min with a significantly low cost. All reactions of metal removal that occurred throughout this reactor obey the kinetic of a first-order reaction. Thermodynamics parameters of present electrocoagulation removal of heavy metals indicate an endothermic, spontaneous nature, and random irregularity at the liquid-solid interaction. The present design of the EC reactor proved the ability of the electrocoagulation process to eliminate heavy metals from wastewater with low amounts of energy and electrode consumption.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability
The data that has been used is confidential.